1. Field of the Invention
The present invention relates to a III-V compound semiconductor device formed on a silicon monocrystalline semiconductor substrate (referred to as Si-substrate, hereinafter) and, in particular, an optical semiconductor device formed on the Si-substrate.
2. Description of Related Art
Growth technology for monocrystalline III-V compound semiconductor layers on a surface of the Si-substrate by using a molecular beam epitaxy (referred to as MBE, hereinafter) or a metal organic vapor phase epitaxy (referred to as MOVPE, hereinafter) is important, in view of cost and integration of two devices such as an optical semiconductor device and silicon integrated circuits on the same Si-substrate.
Referring to FIG. 1, a conventional semiconductor laser device formed on a Si-substrate has a first and second buffer layers 2 and 4 sandwiching a strain superlattice layer 3 of InP-In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y (0&lt;x&lt;1, 0&lt;y&lt;1). The first buffer layer 2 comprises an n-type GaAs of 1-3 .mu.m thickness. The second buffer layer 4 comprises an n-type InP of 10-20 .mu.m thickness. On the second buffer layer 4, an n-type cladding layer 5 of In.sub.u Ga.sub.1-u As.sub.w P.sub.1-w (2.0 .mu.m thick), a p-type active layer 6 of In.sub.0.76 Ga.sub.0.24 As.sub.0.55 P.sub.0.45, a p-type active layer 7 of In.sub.u Ga .sub.1-u As.sub.w P.sub.1-w (1.5 .mu.m thick) and a n-type contact layer 8 of In.sub.t Ga.sub.1-t As.sub.u P.sub.1-u (1.0 .mu.m thick) are grown in that order and a p+-type diffusion layer 10 is formed by diffusing zinc through an opening formed in a silicon oxide film 9 formed on the contact layer 8. Reference numerals 11 and 12 depict a p-side electrode and an n-side electrode, respectively.
When a GaAs layer is grown in a Si-substrate, defects appear owing to lattice mismatch or anti-phased domain. Silicon (Si) is a non-polar semiconductor having a covalent bond, while GaAs is a polar semiconductor having a partially ionic bond. The lattice constant of GaAs is larger than that of Si by 4%. Furthermore, the thermal expansion coefficient of GaAs is larger than that of Si.
If an initial layer on the Si-substrate is a mixed layer of gallium (Ga) or arsenic (As) atoms, the anti-phased domain is formed. The anti-phased domain is prevented by forming a single atom layer of either As or Ga before the GaAs growth is started. However, since the difference in lattice constant between Si and GaAs is large, dislocation of about 10.sup.12 -10.sup.13 /cm.sup.2 is concentrated on a flat interface.
Although the influence of edge type dislocation defined in this interface region on reliability of transistors and/or laser diode formed on the interface is not known sufficiently, growth of dislocation, that is, upward propagation thereof, causes leak current to be increased which leads to degradation of semiconductor device performance.
In order to avoid the adverse influence due to the upward propagation of dislocation, the first and second buffer layers 2 and 4 are made sufficiently thick as mentioned previously.
In the conventional semiconductor device mentioned above, stress is caused due to difference in thermal expansion coefficient between the Si-substrate and the GaAs layer (the first buffer layer), and thus the Si-substrate tends to be warped.
Furthermore, in order to prevent the adverse influence due to the propagation of crystal defect such as dislocation in the interface between the Si-substrate and its vicinity of the interface, the first and second buffer layers, particularly, the second buffer layer must be sufficiently thick. Therefore, the crystal growth process requires more than ten hours which limits the production volume of the semiconductor device. Moreover, since the thickness of the whole semiconductor device becomes large, heat dissipation through the substrate is decreased.